Hmm.... you're right about the rotor spinning, etc.
Maybe that mark is more of a manufacturing mark, like, two different 'brands' might have been available. One with the little "pac-man" mark as shown in the dotted circle, and a different one with a different-shaped mark. And maybe they're not interchangeable...
Or, maybe the generator looks like it can be detached from the half-moon shaped mounting plate. Maybe the mark serves to 'orient' the whole assembly. Or, in that picture, it looks like the generator is adjustable (the slotted bit just visible in the picture) and the marks serve to orient it all.
That's about all I can come up with...
Good luck with it.
Kirk